The present invention generally pertains to patient monitoring using photoplethysmographic devices to generate blood analyte information. More particularly, the invention is directed to an efficient and effective approach for reducing the undesired effects of motion-contaminated data in pulse oximetry devices.
In the field of photoplethysmography light signals corresponding with two or more different centered wavelengths may be employed to non-invasively determine various blood analyte concentrations. By way of primary example, blood oxygen saturation (SpO2) levels of a patient""s arterial blood are monitored in pulse oximeters by measuring the absorption of oxyhemoglobin and reduced hemoglobin using red and infrared light signals. The measured absorption data allows for the calculation of the relative concentrations of reduced hemoglobin and oxyhemoglobin, and therefore SpO2 levels, since reduced hemoglobin absorbs more light than oxyhemoglobin in the red band and oxyhemoglobin absorbs more light than reduced hemoglobin in the infrared band, and since the absorption relationship of the two analytes in the red and infrared bands is known.
To obtain absorption data, pulse oximeters comprise a probe that is releaseably attached to a patient""s appendage (e.g., finger, ear lobe or the nasal septum). The probe directs red and infrared light signals through the appendage, or tissue-under-test. The light signals are provided by one or more sources which are typically disposed in the probe. A portion of the light signals is absorbed by the tissue-under-test and the intensity of the light transmitted through the tissue-under-test is detected, usually by at least one detector that may be also located in the probe. The intensity of an output signal from the detector(s) is utilized to compute SpO2 levels, most typically via a processor located in a patient monitor interconnected to the probe.
As will be appreciated, pulse oximeters rely on the time-varying absorption of light by a tissue-under-test as it is supplied with pulsating arterial blood. The tissue-under-test may contain a number of non-pulsatile light absorbers, including capillary and venous blood, as well as muscle, connective tissue and bone. Consequently, detector output signals typically contain a large non-pulsatile, or DC, component, and a relatively small pulsatile, or AC, component. It is the small pulsatile, AC component that provides the time-varying absorption information utilized to compute arterial SpO2 levels.
In this regard, the red and infrared signal portions of pulse oximeter detector output signals each comprise corresponding large DC and relatively small AC components. The red and infrared signal portions have an exponential relationship to their respective incident intensities at the detector(s). As such, the argument of the red and infrared signal portions have a linear relationship and such portions can be filtered and processed to obtain a ratio of processed red and infrared signal components (e.g., comprising their corresponding AC and DC components), from which the concentration of oxyhemoglobin and reduced hemoglobin in the arterial blood may be determined. See, e.g., U.S. Pat. No. 5,934,277. By utilizing additional light signals at different corresponding centered wavelengths it is also known that carboxyhemoglobin and methemoglobin concentrations can be determined. See, e.g., U.S. Pat. No. 5,842,979.
As noted, the pulsatile, AC component of a pulse oximeter detector output signal is relatively small compared to the non-pulsatile DC component. Consequently, the accuracy of analyte measurements can be severely impacted by small amounts of noise. Of particular concern here is noise that contaminates absorption data as a result of undesired variations in the path length of light signals as they pass through the tissue-under-test. Such variations are most typically caused by patient movement of the appendage to which a pulse oximetry probe is attached.
A number of different approaches have been utilized to reduce the deleterious effects of patient motion in pulse oximeters. For example, pulse oximeter probes have been developed to enhance the physical interface between the probe and tissue-under-test, including the development of various clamp type probe configurations and secure wrap-type probe configurations. Further, numerous approaches have been developed for addressing motion contaminated data through data processing techniques. While such processing techniques have achieved a degree of success, they often entail extensive signal processing requirements, thereby contributing to increased device complexity and componentry costs.
In view of the foregoing, a general objective of the present invention is to provide an improved method and system for addressing motion correction in pulse oximeters.
More particularly, primary objectives of the present invention are to provide for motion correction in pulse oximeters in a manner that is effective and that reduces complexity and component requirements relative to data processing-intensive prior art devices.
The above objectives and additional advantages are realized in the present invention. In this regard, the present inventor has recognized that patient motion can be generally classified into a limited number of motion ranges, or bands, for motion correction purposes. A first type of motion, which may be referred to as xe2x80x9clow motionxe2x80x9d, corresponds with a range of patient motions that do not have a significant effect on the absorption data comprising a detector output signal. A second type of motion, which may be referred to as xe2x80x9cclinical motionxe2x80x9d, corresponds with patient motion that primarily affects the AC component of a detector output signal in a relatively predictable manner across a range of motion severity. For such clinical motion, the present inventor has further recognized that a predetermined adjustment factor may be effectively utilized to correct blood analyte measurements, thereby avoiding complex data processing requirements.
The above-noted recognitions provide the basis for a number of improvements to pulse oximetry systems in which a detector provides an output signal indicative of light absorption of a tissue-under-test at each of a plurality of different centered light wavelengths (e.g. centered at red and infrared wavelengths) and which utilize the output signal to obtain blood analyte indicator values for each of a succession of measurements (e.g., periodic measurements corresponding with partially overlapping or non-overlapping measurement data) during a patient monitoring procedure. The inventive method/system provides for the utilization of the detector output signal to obtain a corresponding relative motion estimate value for each measurement. For such purposes, the detector output signal may be processed to yield a different plurality of differential absorption data sets for each measurement, wherein each data set includes a first differential absorption value for light at the first wavelength dAxcex1 (e.g., infrared light) and a corresponding-in-time second differential absorption value for light at the second wavelength dAxcex2(e.g., infrared light). As will be appreciated, the plurality of data sets corresponding with each given measurement are obtained over an associated time period, wherein each successive measurement employs a different successive plurality of data sets, and wherein the data sets employed for successive measurements may or may not partially overlap. By way of primary example, the differential absorption data sets may be obtained via derivative or logarithmic processing of a series of data samples that correspond with a detected infrared portion of a detector output signal and a corresponding-in-time series of data samples that correspond with a detected red light portion of the detector output signal.
For each of the measurements the method/system provides for a determination of whether or not the corresponding relative motion estimate value (RMEV) is within a first predetermined range (e.g. corresponding with a predetermined range of clinical motion), wherein for measurements having a corresponding RMEV within the first predetermined range the corresponding blood analyte indicator value (BAIV) may be adjusted in a predetermined manner. Such adjustment may provide for the use of a predetermined adjustment factor (e.g., determined empirically). For measurements indicating clinical motion, the adjusted BAIVs may be utilized to obtain a more accurate measure of blood analyte concentration (e.g. the relative concentration of oxygenated hemoglobin and reduced hemoglobin for SpO2 level determination). As will be appreciated, one or more clinical motion bands, with corresponding predetermined RMEV ranges and adjustment factors, may be employed in accordance with the present invention.
As noted, the present inventor has recognized that for a certain range of patient motion, referred to as low motion herein, the impact of the motion on blood analyte measurements is negligible. As such, the inventive method/system may further provide for a determination as to whether the relative motion estimate value (RMEV) for each of the plurality of measurements is within a second predetermined range (e.g., corresponding with low motion). For each measurement having a corresponding RMEV within the second predetermined range, the corresponding blood analyte indicator value may be employed without adjustment to obtain blood analyte concentration values.
The RMEV obtained with respect to each given measurement period may entail the computation of a best-fit function for the corresponding plurality of differential absorption data sets. By way of example, the best-fit function may be defined as the slope value of a regression line (e.g. determined via least squares or linear regression processing) for a xe2x80x9cplotxe2x80x9d of the differential absorption values for light at the first wavelength dAxcex1 versus the corresponding-in-time differential absorption values for light at the second wavelength dAxcex2. As may be appreciated, the best fit function obtained with respect to each measurement period may also be utilized as the corresponding BAIV, e.g., the xe2x80x9cbest-fit RRatioxe2x80x9d value as taught in U.S. Pat. No. 5,954,277. Alternatively, a first plurality of differential absorption data sets may be computed for use in the computation of the RMEV; and a second plurality of differential absorption data sets may be separately computed for use in the determination of the BAIVs as taught in a co-pending U.S. patent application entitled xe2x80x9cMethod And Apparatus For Determining Pulse Oximetry Differential Valuesxe2x80x9d, filed contemporaneous herewith and hereby incorporated by reference.
In conjunction with the obtainment of a best fit function for each given measurement, the inventive method/system may further provide for the performance of a statistical analysis of the corresponding plurality of a differential absorption data sets in relation to the best fit function to obtain at least one statistical variance value indicative of a degree of any associated motion, wherein the statistical variance value(s) may be utilized to compute the corresponding RMEV. In primary embodiments, the statistical analysis may comprise a principal component analysis (PCA). That is, for each measurement the plurality of differential absorption data sets (i.e., each comprising corresponding-in-time dAxcex1 and dAxcex2 values) may be statistically analyzed in relation to the corresponding best fit function to obtain at least one of a first principal component variance value (V1) and a second principal component variance value (V2). In this regard, V1 may be computed to represent an amplitude-based statistical variance, while V2 may be computed to represent a scatter-based statistical variance. The relative motion estimate value for a given measurement may be determined utilizing at least one, and preferably both, of the corresponding first and second principal component variance values V1 and/or V2.
More particularly, in one embodiment, for each given one of a plurality of measurements the inventive method/system may provide for the calculation of a corresponding current motion estimate value (CMEV) utilizing the corresponding V1 and V2 values, and the identification of a reference motion value (RMV), wherein the corresponding relative motion estimate value (RMEV) is computed utilizing both the CMEV and RMV. In one arrangement, the CMEV for each given measurement may be calculated as follows:
CMEV=V1*V2
In turn, the RMEV for each given measurement may be calculated as a ratio of the corresponding CMEV and RMV, e.g.:   RMEV  =            CMEV      RMV        .  
The RMV may be established on an ongoing, updated basis to be equal to the RMEV that corresponds with the lowest amount of motion (e.g., the lowest RMEV) determined with respect to any prior measurement (e.g. preferably corresponding low motion) for a given patient monitoring procedure (i.e., a xe2x80x9clow motion referencexe2x80x9d). At the outset of a given measurement procedure and/or where there is otherwise no prior low motion reference basis for establishing an RMV, the RMV for the given measurement may be established as follows:       RMV    =                            V          1                          V          2                    ⁢              xe2x80x83            ⁢              CMEV        *            ⁢      K        ,
where V1, V2 and CMEV are as determined with respect to the given current measurement and K is a constant.
As indicated, for measurements having an RMEV within a first predetermined range, i.e., corresponding with clinical motion, a predetermined adjustment factor (PAF) may be utilized to adjust the corresponding blood analyte indicator value (BAIV) for use in blood analyte concentration computations. Such adjustment may entail the ready application, e.g., by multiplication, of the PAF to the computed BAIV. In this regard, the PAF may be empirically set via statistical analysis of clinical motion-affected absorption data and corresponding-in-time, non-motion-affected absorption data obtained via testing of test subject control groups.
In one embodiment, for measurements having computed BAIVs that exceed a predetermined threshold value, the PAF may be scaled in relation to the predetermined threshold value. For example, in applications where each BAIV is defined by the slope value of a regression line, the PAF for a given BAIV may be scaled follows:             Scaled      ⁢              xe2x80x83            ⁢      PAF        =          1      -              [                              [                                          BAIV                -                A                                            B                -                A                                      ]                    *                      (                          1              -              PAF                        )                          ]              ,
wherein A and B are predetermined constants, and wherein no scaling occurs when:
BAIV greater than B,
and wherein PAF may be set to 1 when:
BAIV less than A.
In arrangements where the BAIVs are defined by regression line slope values as noted above, the PAF may be preferably set between about 0.5 and 0.85, and most preferably between about 0.6 and 0.75 (e.g., about 0.6875 in one arrangement where A=0.9455 and B=0.65).
In additional aspects of the present invention, the inventive method/system may further comprise additional features to insure appropriate motion correction where there has been a rapid, significant change in the perfusion of a tissue-under-test. By way of example, such variations may occur as a result of the application of a tourniquet or blood pressure cuff, or as a result of inadvertent contact with a patient xe2x80x9cpressure pointxe2x80x9d (e.g., that cuts off arterial blood flow to the tissue under test). To address such instances the inventive method/system may provide for the ongoing, periodic determination of a motion probability factor (MPF), wherein if the MPF exceeds a predetermined threshold the reference motion value (RMV) may be adjusted using the MPF.
The MPF may be computed via a statistical analysis of the plurality of differential absorption data sets corresponding with each of all given one of a series of measurements in relation to a corresponding best fit function computed therefore, wherein at least one statistical variance value may be obtained for use as an MPF value. More particular, the MPF may be computed utilizing of V2 and/or V1 values computed with respect to each current a series of measurements and computed with respect to one or more prior measurements. In one approach, an average V2 value is determined in relation to each given measurement, wherein each average V2 value is calculated by averaging the sum of the V2 value for a given period and the V2 values for a predetermined number of immediately precedent measurements. Then, an MPF for each given measurement may be computed utilizing a comparison, or ratio, between the average V2 value computed for the current measurement and the lowest average V2 value computed with respect to any prior measurement (e.g., current average V2 value/lowest average V2 value).
In one arrangement, the system/method may be established so that an RMV adjustment may be made when (i) a predetermined number of successive, non-low motion measurements have occurred, and (ii) the motion probability factor (MPF) for the current measurement period is determined to exceed a predetermined threshold. When the predetermined threshold is exceeded, the RMV may be adjusted as follows:
adjusted RMV=CMEV+(CMEVxe2x88x92RMV)(MPF)K,
wherein CMEV and MPF are as determined with respect to the current measurement and K is a constant.
In addition to the implementation of one or more clinical motion ranges and a low motion range, embodiments of the present invention may also address patient motion that exceeds clinical motion and may be referred to as xe2x80x9csevere motionxe2x80x9d. In such embodiments, the present invention accommodates an approach wherein no blood analyte measurement values are output to a user during xe2x80x9csevere motionxe2x80x9d (e.g., when measurements have corresponding RMEVs outside of the predetermined clinical motion and low motion RMEV ranges).
Alternatively, to address severe motion embodiments may be employed which provide for the use and adjustment of a previously determined blood analyte indicator value (BAIV). For example, the BAIV for the most recent non-severe motion measurement may be adjusted by an adjustment factor for use in blood analyte concentration computations. The adjustment factor may be based upon tracking of the DC components of the detector output signal portion(s) corresponding with the detected light at the first and/or second centered-wavelength(s) signals. That is, the adjustment factor may be computed as the ratio between such DC component(s) measurement, and such DC components corresponding with at least a current, given severe motion measurement (i.e. a measurement obtained utilizing data obtained during severe motion.
In this regard, in one embodiment the detector output signal may be utilized to obtain moving average values of total tissue absorption of the AC and DC signal components corresponding with each of the first and second light signals (MAVxcex1, MAVxcex2), e.g., for red and infrared signal components, wherein such moving average values are computed for a predetermined precedent time interval in conjunction with each measurement (e.g., an average for a predetermined number of measurements that include the current measurement and a number of prior measurements). In the event severe motion is detected with respect to a given measurement period, e.g. when the corresponding RMEV is not within a predetermined low motion range or any predetermined clinical motion range, the BAIV corresponding with the most recent, non-severe motion measurement may be adjusted by a DC tracking factor (DCTF), as follows:       DCTF    =                                                      MAV                              λ                ⁢                                  xe2x80x83                                ⁢                2                                                    MAV                              λ                ⁢                                  xe2x80x83                                ⁢                1                                              ⁢                      xe2x80x83                    ⁢          for          ⁢                      xe2x80x83                    ⁢          most          ⁢                      xe2x80x83                    ⁢          recent          ⁢                      xe2x80x83                    ⁢          pre          ⁢                      -                    ⁢          severe          ⁢                      xe2x80x83                    ⁢          motion          ⁢                      xe2x80x83                    ⁢          measurement                                                    MAV                              λ                ⁢                                  xe2x80x83                                ⁢                2                                                    MAV                              λ                ⁢                                  xe2x80x83                                ⁢                1                                              ⁢                      xe2x80x83                    ⁢          for          ⁢                      xe2x80x83                    ⁢          current          ⁢                      xe2x80x83                    ⁢          severe          ⁢                      -                    ⁢          motion          ⁢                      xe2x80x83                    ⁢          measurement                    ⁢              xe2x80x83            ⁢      K        ,
wherein K is a constant; then:
adjusted BAIV=BAIV*DCTF.
The adjusted BAIV may then be employed for enhanced blood analyte determinations for the given severe motion measurement. As may be appreciated, this inventive aspect may be advantageously utilized in a number of arrangements, including embodiments where no clinical motion bands are defined/employed for motion correction purposes. For example, this feature may be separately utilized with the system/method disclosed in PCT Publication No. WO 98/04903, PCT Application No. PCT/CH97/00282, hereby incorporated by reference.
Additional aspects and other combinations of the present invention will be apparent to those skilled in the art upon review of the further description that follows.